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<HTML><LINK HREF="style.css" REL="STYLESHEET" TYPE="text/css"><HEAD><TITLE>Bitmaps, Device Contexts and BitBlt</TITLE></HEAD><BODY><FONT SIZE="-1">[ <A HREF="./index.html">contents</A>| <A HREF="http://www.winprog.org/">#winprog</A>]</FONT><HR><H1>Bitmaps, Device Contexts and BitBlt</H1><P>Example: bmp_one</P><IMG SRC="images/bmp_one.gif" ALT="[images/bmp_one.gif]" ALIGN="right"><H2>GDI</H2>The really great thing about MS Windows is that unlike DOS, you don't need to know anythingabout what video hardware you are using to display graphics. Instead, windows providesan API called the Graphics Device Interface, or GDI. The GDI uses a set of generic graphics objects that can be used to draw to the screen, to memory, or even to printers.<H3>Device Contexts</H3>The GDI revolves around an object called the Device Context (DC), represented by the data type <CODE>HDC</CODE> (Handle to Device Context). An HDC is basically a handle to something you candraw on; it can represent the entire screen, an entire window, the client area of a window,a bitmap stored in memory, or a printer. The nice part is that you don't even need to know which one itrefers to, you can use it basically the same way, which is especially handy for writing customdrawing functions which you can then use on any of these devices without changing it for each one.<P>An HDC like most GDI objects is opaque, meaning thatyou can't access it's data directly... but you can pass it to various GDI functions that willoperate on it, either to draw something, get information about it, or change the object insome way.<P>For example, if you wanted to draw on a window, first you would retreive an <CODE>HDC</CODE> representing the window with <CODE>GetDC()</CODE>, then you could use any of the GDI functionsthat take an <CODE>HDC</CODE> like <CODE>BitBlt()</CODE> for drawing images, <CODE>TextOut()</CODE> for drawing text, <CODE>LineTo()</CODE> for lines and so on.<H3>Bitmaps</H3>Bitmaps can be loaded much like icons in earlier examples, there is <CODE>LoadBitmap()</CODE> for the most basic functionality of simply loading a bitmap resource, and <CODE>LoadImage()</CODE> can be used to load bitmaps from a <CODE>*.bmp</CODE> file just as it can for icons.<P>One of the quirks of GDI is that you can't draw to bitmap objects (<CODE>HBITMAP</CODE> type) directly.Remember that drawing operations are abstracted by Device Contexts, so in order to use these drawing functions on a bitmap, you need to create a Memory DC, and then select the <CODE>HBITMAP</CODE> intoit with <CODE>SelectObject()</CODE>. The effect is that the "device" that the <CODE>HDC</CODE> refers to is the bitmap in memory, and when you operateon the <CODE>HDC</CODE>, the resulting graphic operations are applied to the bitmap. As I mentioned, this is actuallya very conveiniant way of doing things, as you can write code that draws to an <CODE>HDC</CODE> and you can use iton a Window DC or a Memory DC without any checks or changes.<P>You do have the option of manipulating the bitmap data in memory yourself. You cando this with Device Independant Bitmaps (DIB), and you can even combine GDI and manual operations on the DIB.However for the time being, this is beyond the scope of the basic tutorial and for now we're just cover thesimpler GDI operations on their own.<H3>GDI Leaks</H3><P>Once you're finished with an <CODE>HDC</CODE>, it's very important to release it (just how you do that dependson how you got it, which we'll talk about in a bit). GDI objects are limited in number. In versions of windows prior to Windows 95, they were not only incredably limited but also shared systemwide, so that if one program used up too many, none of the rest would be able to draw anything!Fortunately this isn't the case any longer, and you could get away with using up quite a lot ofresources in Windows 2000 or XP before anything too bad happened... but it's easy to forget tofree GDI objects and they can quickly run your program out of GDI resources under Windows 9x.Theorehtically you shouldn't be able to drain the system of GDI resources in NT systems (NT/2K/XP)but it still happens in extreme cases, or if you hit the right bug on the nose.<P>If your program runs fine for a few minutes and then starts drawing strangely or not at all, it's a good sign that you're leaking GDI resources. HDCs aren't the only GDI objects you need to be carefulabout releasing, but generally it's ok to keep things like bitmaps and fonts around for the entirelifetime of your program, since it's much more efficiant than reloading them each time you need them.<P>Also, an HDC can only contain one of each type of object (bitmap, font, pen...) at a time, and when youselect a new one in it will return the last one. It's very important that you deal with this objectproperly. If you ignore it completely, it will be lost and they will pile up in memory causing GDIleaks. When an HDC is created, it's also created with some default objects selectedinto it... it's a good idea to store these when they are returned to you, and then when you are completeddrawing with the <CODE>HDC</CODE> select them back into it. This will not only remove any of your own objects from the <CODE>HDC</CODE> (which is a good thing) but it will also cause the default objects to be properlydisposed of when you release or destroy the <CODE>HDC</CODE> (a VERY good thing).<P><B>Important Update:</B> Not all objects have defaults selected into <CODE>HDC</CODE>s, and you can refer to MSDN for the few that don't. Becauseof this I was previously uncertain as to wether <CODE>HBITMAP</CODE>s were one of them, since there doesn't seem to be any definitive documentation on it, and examples (even those by Microsoft) often ignored the default bitmap.Since the writing of the original tutorial several years ago, it was confirmed to me that there was in facta default bitmap that needs releasing. This information is courtesy of Shaun Ivory, a software engineer forMS and a friend of mine from #winprog. <P>Apparently there was a bug in a screensaver written at MS, and itturns out it was because the default bitmap wasn't getting replaced or destroyed, and it eventually ran outof GDI resources. Be warned! It's an easy mistake to make.<H2>Displaying Bitmaps</H2>Ok, down to business. The simplest drawing operations on a window occure by handling <CODE>WM_PAINT</CODE>.When your window is first displayed, restored from being minimised, or uncovered from having another windowon top of it, Windows sends the <CODE>WM_PAINT</CODE> message to the window to let it know that it needsto redraw it's contents. When you draw something on the screen it is NOT permanent, it's only there untillsomething else draws over it, and at that point you need to draw it again when the time comes.<PRE CLASS="SNIP">HBITMAP g_hbmBall = NULL;</PRE><PRE CLASS="SNIP"> case WM_CREATE: g_hbmBall = LoadBitmap(GetModuleHandle(NULL), MAKEINTRESOURCE(IDB_BALL)); if(g_hbmBall == NULL) MessageBox(hwnd, "Could not load IDB_BALL!", "Error", MB_OK | MB_ICONEXCLAMATION); break;</PRE>The first step is of course loading the bitmap, this is quite simple with a bitmap resource, there areno significant differences from loading other resource types. Then we can get down to drawing...<PRE CLASS="SNIP"> case WM_PAINT: { BITMAP bm; PAINTSTRUCT ps; HDC hdc = BeginPaint(hwnd, &ps); HDC hdcMem = CreateCompatibleDC(hdc); HBITMAP hbmOld = SelectObject(hdcMem, g_hbmBall); GetObject(g_hbmBall, sizeof(bm), &bm); BitBlt(hdc, 0, 0, bm.bmWidth, bm.bmHeight, hdcMem, 0, 0, SRCCOPY); SelectObject(hdcMem, hbmOld); DeleteDC(hdcMem); EndPaint(hwnd, &ps); } break;</PRE><H3>Getting the Window DC</H3><P>To start off we declare a couple of variables we need. Notice that the first one is a <CODE>BITMAP</CODE>, not an <CODE>HBITMAP</CODE>. <CODE>BITMAP</CODE> is a struct that holds information about an <CODE>HBITMAP</CODE>which is the actual GDI object. We need a way to get the height and width of the <CODE>HBITMAP</CODE> so weuse <CODE>GetObject()</CODE> which contrary to it's name doesn't really get an object, but rather informationabout an existing one. "GetObjectInfo" would have been a more appropriate label. <CODE>GetObject()</CODE> worksfor various GDI object types which it can distinguish based on the value of the second parameter, the size of thestructure.<P>The <CODE>PAINTSTRUCT</CODE> is a structure that contains information about the window being painted and what exactly isgoing on with the paint message. For most simple tasks, you can simply ignore the information it contains, but it'srequired for the call to <CODE>BeginPaint()</CODE>. <CODE>BeginPaint()</CODE> as it's name suggests is designedspecifically for handling the <CODE>WM_PAINT</CODE> message. When not handling a <CODE>WM_PAINT</CODE> message you would use <CODE>GetDC()</CODE> which we will see in the timer animation examples in a while... but in <CODE>WM_PAINT</CODE>, it's importantto use <CODE>BeginPaint()</CODE> and <CODE>EndPaint()</CODE>.<P><CODE>BeginPaint()</CODE> returns us an <CODE>HDC</CODE> that represents the <CODE>HWND</CODE> that we pass to it, theone that <CODE>WM_PAINT</CODE> is being handled for. Any drawing operation we perform on this <CODE>HDC</CODE> will immediately displayon the screen.<H3>Setting up a Memory DC for the Bitmap</H3><P>As I mention above, in order to draw on or with bitmaps, we need to create a DC in memory... the easiest way to do thathere is to <CODE>CreateCompatibleDC()</CODE> with the one we already have. This gives us a Memory DC that iscompatible with the color depth and display properties of the <CODE>HDC</CODE> for the window.<P>Now we call <CODE>SelectObject()</CODE> to select the bitmap into the DC being careful to store the defaultbitmap so that we can replace it later on and not leak GDI objects.<H3>Drawing</H3><P>Once we've gotten the dimentions of the bitmap filled into the <CODE>BITMAP</CODE> struct, we can call <CODE>BitBlt()</CODE> to copy the image from our Memory DC to the Window DC, thus displaying on the screen.As always, you can look up each parameter in MSDN, but in short they are: The destination, the positionand size, the source and source position, and finally the Raster Operation (ROP code), which specifies how to do the copy. In this case, we want a simple exact copy of the source made, no fancy stuff.<P><CODE>BitBlt()</CODE> is probably the all time happiest function in all of the Win32 API and is the staplediet of anyone learning to write games or other graphics applications in windows. It was probably thefirst API that I memorised all the parameters to.<H3>Cleanup</H3><P>At this point the bitmap should be on the screen, and we need to clean up after ourselves. The first thingto do is restore the Memory DC to the state it was when we got it, which means replacing our bitmap with thedefault one that we saved. Next we can delete it altogether with <CODE>DeleteDC()</CODE>.<P>Finally we release the Window DC we got from <CODE>BeginPaint()</CODE> using <CODE>EndPaint()</CODE>.<P>Destroying an <CODE>HDC</CODE> is a little confusing sometimes because there are at least 3 ways to do itdepending on how you got it in the first place. Here's a list of the common methods of gaining an HDC,and how to release it when you're done.<UL><LI>GetDC() - ReleaseDC()<LI>BeginPaint() - EndPaint()<LI>CreateCompatibleDC() - DeleteDC()</UL><P>And finally, at the termination of our program, we want to free any resources that we allocated. Technicallyspeaking this isn't absolutely required, since modern Windows platforms are pretty good at freeing everythingwhen your program exists, but it's always a good idea to keep track of your own objects because if get lazyand don't delete them they have a habit of getting loose. And no doubt, thereare still bugs in windows especially older versions that won't clean up all of your GDI objects if you don't do a thorough job.<PRE CLASS="SNIP"> case WM_DESTROY: DeleteObject(g_hbmBall); PostQuitMessage(0); break;</PRE><HR><FONT SIZE="-1">Copyright © 1998-2003, Brook Miles (<A HREF="mailto:forger(nospam)winprog.org">theForger</A>). 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